Increased Muscle Glucose Uptake Research Articles

Deficiency in the extracellular signal-regulated kinase 1 (ERK1) protects leptin-deficient mice from insulin resistance without affecting obesity

Extracellular signal-regulated kinase (ERK) activity is increased in adipose tissue in obesity and type 2 diabetes mellitus and strong evidences suggests that it is implicated in the downregulation of insulin signalling and action in the insulin-resistant state. To determine the role of ERK1 in obesity-associated insulin resistance in vivo, we inactivated Erk1 (also known as Mapk3) in obese leptin-deficient mice (ob/ob). Mice of genotype ob/ob-Erk1⁻(/)⁻ were obtained by crossing Erk1⁻(/)⁻ mice with ob/ob mice. Glucose tolerance and insulin sensitivity were studied in 12-week-old mice. Tissue-specific insulin sensitivity, insulin signalling, liver steatosis and adipose tissue inflammation were determined. While ob/ob-Erk1⁻(/)⁻ and ob/ob mice exhibited comparable body weight and adiposity, ob/ob-Erk1⁻(/)⁻ mice did not develop hyperglycaemia and their glucose tolerance was improved. Hyperinsulinaemic-euglycaemic clamp studies demonstrated an increase in whole-body insulin sensitivity in the ob/ob-Erk1⁻(/)⁻ mice associated with an increase in both insulin-stimulated glucose disposal in skeletal muscles and adipose tissue insulin sensitivity. This occurred in parallel with improved insulin signalling in both tissues. The ob/ob-Erk1⁻(/)⁻ mice were also partially protected against hepatic steatosis with a strong reduction in acetyl-CoA carboxylase level. These metabolic improvements were associated with reduced expression of mRNA encoding inflammatory cytokine and T lymphocyte markers in the adipose tissue. Our results demonstrate that the targeting of ERK1 could partially protect obese mice against insulin resistance and liver steatosis by decreasing adipose tissue inflammation and by increasing muscle glucose uptake. Our results indicate that deregulation of the ERK1 pathway could be an important component in obesity-associated metabolic disorders.

Hepatic lipotoxicity and the pathogenesis of nonalcoholic steatohepatitis

A significant body of evidence now forces us to rethink the causes of NASH. Once thought to be a disease caused by triglyceride accumulation in hepatocytes with subsequent oxidant stress and lipid peroxidation causing inflammation and fibrosis, new data from animal studies and a limited number of human studies now provide convincing evidence that triglyceride accumulation does not cause insulin resistance or cellular injury in the liver. The lipotoxic liver injury hypothesis for the pathogenesis of NASH suggests that we need to focus our therapeutic efforts on reducing the burden of fatty acids going to the liver or being synthesized in the liver. This can be accomplished by improving insulin sensitivity at the level of adipose tissue to prevent inappropriate peripheral lipolysis and by preventing unnecessary de novo lipogenesis in the liver. Excess carbohydrates are the major substrates for de novo lipogenesis, and thus, reducing carbohydrate consumption through dietary changes and increasing muscle glucose uptake through exercise remain important cornerstones of treatment and prevention of lipotoxic liver injury, a disease hitherto called NASH.

Skeletal Muscle Metabolism in the Pathology and Treatment of Type 1 Diabetes

Type 1 diabetes is characterised by the absence of circulating insulin due to the autoimmune destruction of beta-cells in the pancreas. Patients are traditionally treated with multiple daily injections of exogenous insulin analogues. However, although these therapies improve quality of life, they are associated with the risk of hypoglycemic episodes and do not prevent the development of debilitating secondary complications. For these reasons, there is increasing demand for new therapies and preventions. One approach is the use of viral or non-viral gene therapy to modify skeletal muscle to produce and secrete insulin into the circulation and/or to increase muscle glucose uptake. Skeletal muscle is a desirable target tissue for the treatment of diabetes not only for its central role in whole body metabolism and glucose homeostasis, but also for its accessibility and amenability to many potential gene therapy technologies. Here, we review the basic metabolic principles of skeletal muscle in the absorptive and post-absorptive states at rest and during exercise and discuss how these processes are affected in type 1 diabetes. Finally, current viral and non-viral strategies for modification of skeletal muscle and their application to the treatment of type 1 diabetes are also presented.

Endogenous Nitric Oxide Contributes to Bradykinin-Stimulated Glucose Uptake but Attenuates Vascular Tissue-Type Plasminogen Activator Release

Bradykinin causes vasodilation, stimulates tissue-type plasminogen activator (t-PA) release and, in rodents, increases muscle glucose uptake. Although bradykinin causes vasodilation partly by activating nitric-oxide synthase (NOS), the role of nitric oxide in regulating bradykinin-stimulated t-PA release is uncertain. This study examined the effect of high-dose NOS inhibition on bradykinin-stimulated t-PA release and glucose uptake in humans. We studied 24 healthy (12 women and 12 men), overweight and obese (body mass index >25 kg/m(2)), normotensive, nondiabetic subjects with normal cholesterol. We measured the effect of intra-arterial N(omega)-monomethyl-L-arginine (L-NMMA, 12 micromol/min) on forearm blood flow (FBF), net t-PA release, and glucose uptake at baseline and in response to intra-arterial bradykinin (50-200 ng/min) in subjects pretreated with the cyclooxygenase inhibitor aspirin. Measurements were repeated after isosorbide dinitrate (ISDN; 5 mg) or sildenafil (50 mg). L-NMMA decreased baseline FBF (P < 0.001), increased baseline forearm vascular resistance (P < 0.001), and increased the t-PA arterial-venous gradient (P = 0.04) without affecting baseline net t-PA release or glucose uptake. During L-NMMA, ISDN tended to decrease baseline net t-PA release (P = 0.06). L-NMMA blunted bradykinin-stimulated vasodilation (P < 0.001 for FBF and FVR). Bradykinin increased net glucose extraction (from -80 +/- 23 to -320 +/- 97 microg/min/100 ml at 200 ng/min bradykinin, P = 0.02), and L-NMMA (-143 +/- 50 microg/min/100 ml at 200 ng/min, P = 0.045) attenuated this effect. In contrast, L-NMMA enhanced bradykinin-stimulated t-PA release (39.9 +/- 7.0 ng/min/100 ml versus 30.0 +/- 4.2 ng/min/100 ml at 200 ng/min, P = 0.04 for L-NMMA). In gender-stratified analyses, L-NMMA significantly increased bradykinin-stimulated t-PA release in women (F = 6.7, P = 0.02) but not in men. Endogenous NO contributes to bradykinin-stimulated vasodilation and glucose uptake but attenuates the fibrinolytic response to exogenous bradykinin.

DHEA improves impaired activation of Akt and PKC ζ/λ‐GLUT4 pathway in skeletal muscle and improves hyperglycaemia in streptozotocin‐induced diabetes rats

Addition of dehydroepiandrosterone (DHEA) to a cultured skeletal muscle locally synthesizes 5alpha-dihydrotestosterone (DHT). It induced activation of glucose metabolism-related signalling pathway via protein kinase B (Akt) and protein kinase C zeta/lambda (PKC zeta/lambda)-glucose transporter-4 (GLUT4) proteins. However, such an effect of DHEA in vivo remains unclear. Using streptozotocin (STZ)-induced rats with type 1 diabetes mellitus, we tested the hypothesis that a single bout of DHEA injection in the rats improves hyperglycaemia and muscle GLUT4-regulated signalling pathway. After 1 week of STZ injection (55 mg kg(-1)) with male Wistar rats, fasting glucose concentrations were determined in a blood sample taken from the tail vein. Blood glucose levels were then monitored for 180 min after DHEA or sesame oil (control) was injected (n = 10 for each group). Blood glucose levels decreased significantly for 30-150 min after 2 mg DHEA injection in the STZ rats. In the skeletal muscle, expression and translocation of GLUT4 protein, phosphorylation of Akt and PKC zeta/lambda, and phosphofructokinase and hexokinase enzyme activities increased significantly by DHEA injection. However, DHEA-induced improvements in Akt and PKC zeta/lambda-GLUT4 pathways were blocked by a DHT inhibitor. These results suggest that a single bout of DHEA injection can improve hyperglycaemia and activate the glucose metabolism-related signalling pathway via Akt and PKC zeta/lambda-GLUT4 proteins of skeletal muscles in rats. Moreover, these results show that a DHEA-induced increase in muscle glucose uptake and utilization might contribute to improvement in hyperglycaemia in type 1 diabetes mellitus.

Muscle Contraction, but Not Insulin, Increases Microvascular Blood Volume in the Presence of Free Fatty Acid–Induced Insulin Resistance

OBJECTIVEInsulin and contraction each increase muscle microvascular blood volume (MBV) and glucose uptake. Inhibiting nitric oxide synthase blocks insulin's but not contraction's effects. We examined whether contraction could augment the MBV increase seen with physiologic hyperinsulinemia and whether free fatty acid (FFA)-induced insulin resistance differentially affects contraction- versus insulin-mediated increases in MBV.RESEARCH DESIGN AND METHODSRats were fasted overnight. Plasma FFAs were increased by intralipid/heparin infusion (3 h), insulin was increased with a euglycemic clamp (3 mU · min−1 · kg−1), and hindlimb muscle contraction was electrically stimulated. Muscle MBV was measured using contrast-enhanced ultrasound. Insulin transport into muscle was measured using 125I-insulin. BQ-123 (0.4 mg/h) was used to block the endothelin-1 (ET-1) receptor A.RESULTSSuperimposing contraction on physiologic hyperinsulinemia increased MBV within 10 min by 37 and 67% for 0.1 or 1 Hz, respectively (P < 0.01). FFA elevation alone did not affect MBV, whereas 0.1 Hz stimulation doubled MBV (P < 0.05) and increased muscle insulin uptake (P < 0.05) despite high FFA. Physiologic hyperinsulinemia during FFA elevation paradoxically decreased MBV (P < 0.05). This MBV decrease was reversed by either 0.1 Hz contraction or ET-1 receptor A antagonism, and the combination raised MBV above basal.CONCLUSIONSContraction recruits microvasculature beyond that seen with physiologic hyperinsulinemia by a distinct mechanism that is not blocked by FFA-induced vascular insulin resistance. The paradoxical MBV decline seen with insulin plus FFA may result from differential inhibition of insulin-stimulated nitric oxide–dependent vasodilation relative to ET-1 vasoconstriction. Our results implicate ET-1 as a potential mediator of FFA-induced vascular insulin resistance.

Ciliary Neurotrophic Factor Stimulates Muscle Glucose Uptake by a PI3-Kinase–Dependent Pathway That Is Impaired With Obesity

OBJECTIVECiliary neurotrophic factor (CNTF) reverses muscle insulin resistance by increasing fatty acid oxidation through gp130-LIF receptor signaling to the AMP-activated protein kinase (AMPK). CNTF also increases Akt signaling in neurons and adipocytes. Because both Akt and AMPK regulate glucose uptake, we investigated muscle glucose uptake in response to CNTF signaling in lean and obese mice.RESEARCH DESIGN AND METHODSMice were injected intraperitoneally with saline or CNTF, and blood glucose was monitored. The effects of CNTF on skeletal muscle glucose uptake and AMPK/Akt signaling were investigated in incubated soleus and extensor digitorum longus (EDL) muscles from muscle-specific AMPKα2 kinase-dead, gp130ΔSTAT, and lean and obese ob/ob and high-fat–fed mice. The effect of C2-ceramide on glucose uptake and gp130 signaling was also examined.RESULTSCNTF reduced blood glucose and increased glucose uptake in isolated muscles in a time- and dose-dependent manner with maximal effects after 30 min with 100 ng/ml. CNTF increased Akt-S473 phosphorylation in soleus and EDL; however, AMPK-T172 phosphorylation was only increased in soleus. Incubation of muscles from AMPK kinase dead (KD) and wild-type littermates with the PI3-kinase inhibitor LY-294002 demonstrated that PI3-kinase, but not AMPK, was essential for CNTF-stimulated glucose uptake. CNTF-stimulated glucose uptake and Akt phosphorylation were substantially reduced in obesity (high-fat diet and ob/ob) despite normal induction of gp130/AMPK signaling—effects also observed when treating myotubes with C2-ceramide.CONCLUSIONSCNTF acutely increases muscle glucose uptake by a mechanism involving the PI3-kinase/Akt pathway that does not require AMPK. CNTF-stimulated glucose uptake is impaired in obesity-induced insulin resistance and by ceramide.

The antidiabetic drug metformin exerts an antitumoral effect in vitro and in vivo through a decrease of cyclin D1 level

Metformin is a widely used antidiabetic agent, which regulates glucose homeostasis through inhibition of liver glucose production and an increase in muscle glucose uptake. Recent studies suggest that metformin may reduce the risk of cancer, but its mode of action in cancer remains not elucidated. We investigated the effect of metformin on human prostate cancer cell proliferation in vitro and in vivo. Metformin inhibited the proliferation of DU145, PC-3 and LNCaP cancer cells with a 50% decrease of cell viability and had a modest effect on normal prostate epithelial cell line P69. Metformin did not induce apoptosis but blocked cell cycle in G(0)/G(1). This blockade was accompanied by a strong decrease of cyclin D1 protein level, pRb phosphorylation and an increase in p27(kip) protein expression. Metformin activated the AMP kinase pathway, a fuel sensor signaling pathway. However, inhibition of the AMPK pathway using siRNA against the two catalytic subunits of AMPK did not prevent the antiproliferative effect of metformin in prostate cancer cells. Importantly, oral and intraperitoneal treatment with metformin led to a 50 and 35% reduction of tumor growth, respectively, in mice bearing xenografts of LNCaP. Similar, to the in vitro study, metformin led to a strong reduction of cyclin D1 protein level in tumors providing evidence for a mechanism that may contribute to the antineoplastic effects of metformin suggested by recent epidemiological studies.

A Role for the Transcriptional Coactivator PGC-1α in Muscle Refueling

The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) has been identified as an inducible regulator of mitochondrial function. Skeletal muscle PGC-1alpha expression is induced post-exercise. Therefore, we sought to determine its role in the regulation of muscle fuel metabolism. Studies were performed using conditional, muscle-specific, PGC-1alpha gain-of-function and constitutive, generalized, loss-of-function mice. Forced expression of PGC-1alpha increased muscle glucose uptake concomitant with augmentation of glycogen stores, a metabolic response similar to post-exercise recovery. Induction of muscle PGC-1alpha expression prevented muscle glycogen depletion during exercise. Conversely, PGC-1alpha-deficient animals exhibited reduced rates of muscle glycogen repletion post-exercise. PGC-1alpha was shown to increase muscle glycogen stores via several mechanisms including stimulation of glucose import, suppression of glycolytic flux, and by down-regulation of the expression of glycogen phosphorylase and its activating kinase, phosphorylase kinase alpha. These findings identify PGC-1alpha as a critical regulator of skeletal muscle fuel stores.

Improved Glucose Homeostasis in Mice with Muscle-Specific Deletion of Protein-Tyrosine Phosphatase 1B

Obesity and type 2 diabetes are characterized by insulin resistance. Mice lacking the protein-tyrosine phosphatase PTP1B in all tissues are hypersensitive to insulin but also have diminished fat stores. Because adiposity affects insulin sensitivity, the extent to which PTP1B directly regulates glucose homeostasis has been unclear. We report that mice lacking PTP1B only in muscle have body weight and adiposity comparable to those of controls on either chow or a high-fat diet (HFD). Muscle triglycerides and serum adipokines are also affected similarly by HFD in both groups. Nevertheless, muscle-specific PTP1B(-/-) mice exhibit increased muscle glucose uptake, improved systemic insulin sensitivity, and enhanced glucose tolerance. These findings correlate with and are most likely caused by increased phosphorylation of the insulin receptor and its downstream signaling components. Thus, muscle PTP1B plays a major role in regulating insulin action and glucose homeostasis, independent of adiposity. In addition, rosiglitazone treatment of HFD-fed control and muscle-specific PTP1B(-/-) mice revealed that rosiglitazone acts additively with PTP1B deletion. Therefore, combining PTP1B inhibition with thiazolidinediones should be more effective than either alone for treating insulin-resistant states.

IL-6 increases muscle insulin sensitivity only at superphysiological levels

Exercise induces an increase in glucose transport in muscle. As the acute increase in glucose transport reverses, it is replaced by an increase in insulin sensitivity. Interleukin-6 (IL-6) increases with exercise and has been reported to activate AMP-activated protein kinase (AMPK). Based on this information, we hypothesized that IL-6 would result in an increase in muscle insulin sensitivity. Rat epitrochlearis and soleus muscles were incubated with 120 ng/ml IL-6. Exposure to IL-6 induced a modest acute increase in glucose transport and was followed 3.5 h later by an increase in insulin sensitivity in epitrochlearis but not soleus muscles. IL-6 also brought about an increase in AMPK phosphorylation in epitrochlearis muscles. We conclude that exposure of fast-twitch muscle to 120 ng/ml IL-6 increases insulin sensitivity by activating AMPK. However, exposure of epitrochlearis muscles to 10 ng/ml IL-6, a concentration >100-fold higher than that attained in plasma during exercise, had no effect on glucose transport or insulin sensitivity. These findings provide evidence that the increases in glucose transport and insulin sensitivity induced by IL-6 are pharmacological rather than physiological effects. We interpret our results as evidence that the increase in IL-6 during exercise does not play a role in the exercise-induced increases in muscle glucose uptake and insulin sensitivity.

Hypoglycemic effect of isoleucine involves increased muscle glucose uptake and whole body glucose oxidation and decreased hepatic gluconeogenesis

Isoleucine, a branched chain amino acid, plays an important role in the improvement of glucose metabolism as evidenced by the increase of insulin-independent glucose uptake in vitro. This study evaluated the effect of isoleucine on glucose uptake and oxidation in fasted rats and on gluconeogenesis in vivo and in vitro. Oral administration of isoleucine decreased the plasma glucose level by 20% and significantly increased muscle glucose uptake by 71% without significant elevation of the plasma insulin level compared with controls at 60 min after administration. Furthermore, expiratory excretion of 14CO2 from [U-14C]glucose in isoleucine-administered rats was increased by 19% compared with controls. Meanwhile, isoleucine decreased AMP levels in the liver but did not affect hepatic glycogen synthesis. Under insulin-free conditions, isoleucine significantly inhibited glucose production when alanine was used as a glucogenic substrate in isolated hepatocytes. This inhibition by isoleucine was also associated with a decline in mRNA levels for phosphoenolpyruvate carboxykinase and glucose-6-phosphatase (G6Pase) and a decreased activity of G6Pase in isolated hepatocytes. These findings suggest that a reduction of gluconeogenesis in liver, along with an increase of glucose uptake in the muscle, is also involved in the hypoglycemic effect of isoleucine. In conclusion, isoleucine administration stimulates both glucose uptake in the muscle and whole body glucose oxidation, in addition to depressing gluconeogenesis in the liver, thereby leading to the hypoglycemic effect in rats.

The Rab GTPase-Activating Protein AS160 Integrates Akt, Protein Kinase C, and AMP-Activated Protein Kinase Signals Regulating GLUT4 Traffic

Insulin-dependent phosphorylation of Akt target AS160 is required for GLUT4 translocation. Insulin and platelet-derived growth factor (PDGF) (Akt activators) or activation of conventional/novel (c/n) protein kinase C (PKC) and 5' AMP-activated protein kinase (AMPK) all promote a rise in membrane GLUT4 in skeletal muscle and cultured cells. However, the downstream effectors linking these pathways to GLUT4 traffic are unknown. Here we explore the hypothesis that AS160 is a molecular link among diverse signaling cascades converging on GLUT4 translocation. PDGF and insulin increased AS160 phosphorylation in CHO-IR cells. Stimuli that activate c/n PKC or AMPK also elevated AS160 phosphorylation. We therefore examined if these signaling pathways engage AS160 to regulate GLUT4 traffic in muscle cells. Nonphosphorylatable AS160 (4P-AS160) virtually abolished the net surface GLUT4myc gains elicited by insulin, PDGF, K(+) depolarization, or 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside but partly, yet significantly, inhibited the effects of 4-phorbol-12-myristate-13-acetate. However, the hypertonicity or 2,4-dinitrophenol-dependent gains in surface GLUT4myc were unaffected by 4P-AS160. RK-AS160 (GTPase-activating protein [GAP] inactive) or 4PRK-AS160 (GAP inactive, nonphosphorylatable) had no effect on surface GLUT4myc elicited by all stimuli. Collectively, these results indicate that activation of Akt, c/n PKC, or alpha2-AMPK intersect at AS160 to regulate GLUT4 traffic, as well as highlight the potential of AS160 as a therapy target to increase muscle glucose uptake.

Chronic Treatment With Sildenafil Improves Energy Balance and Insulin Action in High Fat–Fed Conscious Mice

Stimulation of nitric oxide-cGMP signaling results in vascular relaxation and increased muscle glucose uptake. We show that chronically inhibiting cGMP hydrolysis with the phosphodiesterase-5 inhibitor sildenafil improves energy balance and enhances in vivo insulin action in a mouse model of diet-induced insulin resistance. High-fat-fed mice treated with sildenafil plus L-arginine or sildenafil alone for 12 weeks had reduced weight and fat mass due to increased energy expenditure. However, uncoupling protein-1 levels were not increased in sildenafil-treated mice. Chronic treatment with sildenafil plus L-arginine or sildenafil alone increased arterial cGMP levels but did not adversely affect blood pressure or cardiac morphology. Sildenafil treatment, with or without l-arginine, resulted in lower fasting insulin and glucose levels and enhanced rates of glucose infusion, disappearance, and muscle glucose uptake during a hyperinsulinemic (4 mU x kg(-1) x min(-1))-euglycemic clamp in conscious mice. These effects occurred without an increase in activation of muscle insulin signaling. An acute treatment of high fat-fed mice with sildenafil plus l-arginine did not improve insulin action. These results show that phosphodiesterase-5 is a potential target for therapies aimed at preventing diet-induced energy imbalance and insulin resistance.

Hypoglycemic effect of isoleucine involves increased muscle glucose uptake and whole body glucose oxidation, and decreased hepatic gluconeogenesis

Hypoglycemic effect of isoleucine involves increased muscle glucose uptake and whole body glucose oxidation, and decreased hepatic gluconeogenesis

Inhalation of Human Insulin (Exubera) Augments the Efficiency of Muscle Glucose Uptake In Vivo

This study assessed the site of increased glucose uptake resulting from insulin inhalation, quantified its effect under steady-state glucose concentrations, and identified the time to onset of effect. Human insulin was administered to 13 beagles via inhalation (Exubera [insulin human (rDNA origin)] Inhalation Powder; n = 7) or infusion into the inferior vena cava (Humulin R; n = 6) using an algorithm to match plasma insulin levels and kinetics for both groups. Somatostatin and glucagon were infused. Glucose was delivered into the portal vein (4 mg x kg(-1) x min(-1)) and a peripheral vein, as needed, to maintain arterial plasma glucose levels at 180 mg/dl. Hepatic exposure to insulin and glucose and liver glucose uptake were similar in both groups. Despite comparable arterial insulin and glucose levels, hind-limb glucose uptake increased 2.4-fold after inhalation compared with infusion due to increased muscle glucose uptake. Glucose infusion rate, nonhepatic glucose uptake, and tracer-determined glucose disposal were about twice as great compared with intravenous insulin. The effect appeared after 1 h, persisting at least as long as arterial insulin levels remained above basal. Pulmonary administration of insulin increases nonhepatic glucose uptake compared with infusion, and skeletal muscle is the likely site of that effect.

Skeletal Muscle-Selective Knockout of LKB1 Increases Insulin Sensitivity, Improves Glucose Homeostasis, and Decreases TRB3

LKB1 is a tumor suppressor that may also be fundamental to cell metabolism, since LKB1 phosphorylates and activates the energy sensing enzyme AMPK. We generated muscle-specific LKB1 knockout (MLKB1KO) mice, and surprisingly, found that a lack of LKB1 in skeletal muscle enhanced insulin sensitivity, as evidenced by decreased fasting glucose and insulin concentrations, improved glucose tolerance, increased muscle glucose uptake in vivo, and increased glucose utilization during a hyperinsulinemic-euglycemic clamp. MLKB1KO mice had increased insulin-stimulated Akt phosphorylation and a > 80% decrease in muscle expression of TRB3, a recently identified Akt inhibitor. Akt/TRB3 binding was present in skeletal muscle, and overexpression of TRB3 in C2C12 myoblasts significantly reduced Akt phosphorylation. These results demonstrate that skeletal muscle LKB1 is a negative regulator of insulin sensitivity and glucose homeostasis. LKB1-mediated TRB3 expression provides a novel link between LKB1 and Akt, critical kinases involved in both tumor genesis and cell metabolism.

L-Arginine infusion increases glucose clearance during prolonged exercise in humans

Nitric oxide synthase (NOS) inhibition has been shown in humans to attenuate exercise-induced increases in muscle glucose uptake. We examined the effect of infusing the NO precursor L-arginine (L-Arg) on glucose kinetics during exercise in humans. Nine endurance-trained males cycled for 120 min at 72+/-1% Vo(2 peak) followed immediately by a 15-min "all-out" cycling performance bout. A [6,6-(2)H]glucose tracer was infused throughout exercise, and either saline alone (Control, CON) or saline containing L-Arg HCL (L-Arg, 30 g at 0.5 g/min) was confused in a double-blind, randomized order during the last 60 min of exercise. L-Arg augmented the increases in glucose rate of appearance, glucose rate of disappearance, and glucose clearance rate (L-Arg: 16.1+/-1.8 ml.min(-1).kg(-1); CON: 11.9+/- 0.7 ml.min(-1).kg(-1) at 120 min, P<0.05) during exercise, with a net effect of reducing plasma glucose concentration during exercise. L-Arg infusion had no significant effect on plasma insulin concentration but attenuated the increase in nonesterified fatty acid and glycerol concentrations during exercise. L-Arg infusion had no effect on cycling exercise performance. In conclusion, L-Arg infusion during exercise significantly increases skeletal muscle glucose clearance in humans. Because plasma insulin concentration was unaffected by L-Arg infusion, greater NO production may have been responsible for this effect.

Sixteen hours of fasting differentially affects hepatic and muscle insulin sensitivity in mice

Fasting readily induces hepatic steatosis. Hepatic steatosis is associated with hepatic insulin resistance. The purpose of the present study was to document the effects of 16 h of fasting in wild-type mice on insulin sensitivity in liver and skeletal muscle in relation to 1) tissue accumulation of triglycerides (TGs) and 2) changes in mRNA expression of metabolically relevant genes. Sixteen hours of fasting did not show an effect on hepatic insulin sensitivity in terms of glucose production in the presence of increased hepatic TG content. In muscle, however, fasting resulted in increased insulin sensitivity, with increased muscle glucose uptake without changes in muscle TG content. In liver, fasting resulted in increased mRNA expression of genes promoting gluconeogenesis and TG synthesis but in decreased mRNA expression of genes involved in glycogenolysis and fatty acid synthesis. In muscle, increased mRNA expression of genes promoting glucose uptake, as well as lipogenesis and beta-oxidation, was found. In conclusion, 16 h of fasting does not induce hepatic insulin resistance, although it causes liver steatosis, whereas muscle insulin sensitivity increases without changes in muscle TG content. Therefore, fasting induces differential changes in tissue-specific insulin sensitivity, and liver and muscle TG contents are unlikely to be involved in these changes.

Control of Exercise-stimulated Muscle Glucose Uptake by GLUT4 Is Dependent on Glucose Phosphorylation Capacity in the Conscious Mouse

Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during exercise because GLUT4 overexpression does not increase exercise-stimulated MGU. Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair exercise-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity was increased. Thus, C57BL/6J mice with hexokinase II (HKII) overexpression (HK(Tg)), a GLUT4 partial knock-out (G4(+/-)), or both (HK(Tg) + G4(+/-)) and wild-type (WT) littermates were implanted with carotid artery and jugular vein catheters for sampling and infusions at 4 months of age. After a 7-day recovery, 5-h fasted mice remained sedentary or ran on a treadmill at 0.6 mph for 30 min (n = 9-12 per group) and received a bolus of 2-deoxy[3H]glucose to provide an index of MGU (Rg). Arterial blood glucose and plasma insulin concentrations were similar in WT, G4(+/-), HKTg, and HKTg + G4(+/-) mice. Sedentary Rg values were the same in all genotypes in all muscles studied, confirming that glucose transport is a significant barrier to basal glucose uptake. Gastrocnemius and soleus Rg were greater in exercising compared with sedentary mice in all genotypes. During exercise, G4(+/-) mice had a marked increase in blood glucose that was corrected by the addition of HK II overexpression. Exercise Rg (micromol/100g/min) was not different between WT and G4(+/-) mice in the gastrocnemius (24 +/- 5 versus 21 +/- 2) or the soleus (54 +/- 6 versus 70 +/- 7). In contrast, the enhanced exercise Rg observed in HKTg mice compared with that in WT mice was absent in HKTg + G4(+/-) mice in both the gastrocnemius (39 +/- 7 versus 22 +/- 6) and the soleus (98 +/- 13 versus 65 +/- 13). Thus, glucose transport is not a significant barrier to exercise-stimulated MGU despite a 50% reduction in GLUT4 content when glucose phosphorylation capacity is normal. However, when glucose phosphorylation capacity is increased by HK II overexpression, GLUT4 availability becomes a marked limitation to exercise-stimulated MGU.